Structures of the straight and branched poly-LacNAc chains. The i antigen is characterized by the straight chain of repeating LacNAc units and is converted to the I antigen structure, characterized by the branched poly-LacNAc chain, through the activity of I β6GlcNAcT, which synthesizes the decisive GlcNAcβ1-6 branch. Various glyco-epitopes, including blood group A, B, and H antigens and the Lewis antigen series (Le^a, Le^b, Le^x, Le^y, sLe^a, and sLe^x), are constructed on the terminal ends of the straight and branched poly-LacNAc chains through different enzymatic pathways.